CHAP. LXVII SUBGLACIAL FELL-FIELD FORMATIONS 257 



We must subdivide subglacial fell-fields into several, probably many, 

 formations ; but they have not yet been sufficiently investigated in low 

 latitudes to render possible accurate scientific subdivision. 



On high mountains in Europe and in arctic countries fell-fields prevail 

 where the mean temperature of the warmest month is below 6 C. At 

 a somewhat lower altitude or latitude fell-field is confined to edaphically 

 unfavourable localities, while plants on more favourable sites produce 

 a closed formation. Low temperature in summer favours the production 

 of raw humus where the vegetation is somewhat richer, and in such places 

 there arise various formations belonging to sour soil, such as moor, heath, 

 moss-tundra, and the like, which have already been dealt with. 



In tropical places the vegetation shows character essentially different 

 from that of the arctic-alpine fell-field, and may provisionally be termed 

 tropical-alpine fell-field. Here, too, may be placed the vegetation of 

 high mountains in very dry countries that recalls steppe-vegetation in 

 many respects. It may be termed mountain-steppe. 



In depressions lying within the subglacial tract where snow remains 

 for a long time, one finds characteristic, greasy mud, which sustains a 

 vegetation of its own Ottli's 1 snow-patch flora. The oecological 

 conditions of this habitat are : (i) The characteristic soil, which owes 

 its origin to melting of the snow ; (2) Brevity of the vegetative season ; 

 (3) Humidity. 



Arctic Fell-field. 



Arctic fell-field occurs round the North Pole in northernmost North 

 America, Siberia, northern Europe, Greenland, Iceland (where it is 

 termed ' melur '). 2 The most important shrubs, dwarf -shrubs, and 

 prostrate and ' turf ' shrubs, are Juniperus communis, many species of 

 Salix, Betula nana, Empetrum ; among Ericaceae, Cassiope tetragona, 

 Arctostaphylos alpina, Loiseleuria procumbens, Rhododendron lapponi- 

 cum, Phyllodoce caerulea^ Vaccinium, Ledum, and Kalmia ; and the 

 rosaceous Dry as octopetala. 3 The most important herbaceous genera 

 are Poa, Festuca, Trisetum, Hierochloe, Nardus, and others among 

 grasses ; Carex, Elyna (E. Bellardi), and Kobresia (K. bipartita), among 

 sedges ; Luzula and Juncus, among Juncaceae ; and the liliaceous 

 Tofieldia. In addition there occur many Caryophyllaceae, including 

 Silene acaulis and Viscaria alpina ; Compositae ; Cruciferae, including 

 Draba, Cochlearia, Vesicaria, and Braya ; Campanula uniflora, Papaver 

 nudicaule, Polygonum viviparum, Pyrola rotundifolia, Rhodiola rosea, 

 also species of Ranunculus, Potentilla, Saxifraga, and Pedicularis. More- 

 over, there are always many mosses and lichens of various forms, including 

 such fruticose lichens as Cetraria, Cornicularia, Sphaerophoron, and Cla- 

 donia ; these Cryptogamia in many places play the leading part or reign 

 almost alone. 



In different stations, according as the soil is more gravelly, clayey, 

 or sandy, more warm or cold, certain species sometimes occur in greater 

 abundance and give a special appearance to the vegetation, so that it is 

 possible to distinguish various associations, such as Juncus trifidus- 

 association, Diapensia-association, Carex rupestris-association, Dryas- 



1 Ottli, 1903 ; C. Schroter, 1904-8. * Stefansson, 1894. 



3 Warming, 18870, 1908 a. 



WARMING S 



